Abstract Legionella pneumophila is a Gram-negative bacteria that replicates inside protozoa in the environment but can also replicate within alveoloar macrophages, thereby causing a pneumonia-like disease called Legionnaires? disease. L. pneumophila is able to survive and replicate within normally bactericidal phagocytic cells by altering their endocytic pathway and preventing rapid phagosome-lysosome fusion. L. pneumophila perturbs normal host cell function by injecting a large number of virulence factors into the host cell cytoplasm via a type IVB secretion system (T4BSS). The L. pneumophila T4BSS is encoded by twenty- seven dot/icm genes, which encode a large macromolecular complex in the bacterial cell envelope. The Dot/Icm system is absolutely required for intracellular growth and virulence. In contrast, the majority of the ~300 Dot/Icm substrates appear to be dispensable for both traits due to multiple levels of redundancy. Therefore, the L. pneumophila Dot/Icm represents the most viable target for therapeutic intervention. Although a significant amount of information has been acquired about the assembly and function of the Dot/Icm system, many questions remain. This proposal will follow-up on a number of recent advances on the Dot/Icm T4BSS. These include the discovery that the Dot/Icm system is located at the poles of the bacterium and polar export of substrates by this specialized secretion system is required for proper intracellular targeting of the LCV (Legionella containing vacuole). The second major advance was the determination of the structure of the c-terminus of DotL bound to the secretion chaperones. The final advance was the acquisition of the first image of this secretion apparatus by cryoEM. Overall this research relates to human health in several ways. First, establishing the mechanism used by L. pneumophila to survive and replicate inside macrophages will provide additional insight into how it causes disease and may reveal novel targets to be used for drug therapy. Second, since specialized secretion systems are commonly used by a variety of bacterial pathogens, knowledge gained about the L. pneumophila secretion apparatus is likely to be applicable to understanding the molecular mechanisms of virulence used by other pathogens, and could serve as the basis to prevent or treat a number of different diseases.